Complexes of polysaccharides or polysaccharide derivatives with magnetic metal oxides are proposed in order to improve stability and toxicity of aqueous magnetic sols. Disclosed in, for example, Japanese Patent Publication 13521/1984 (U.S. Pat. No. 4,101,435) are a complex of dextran or dextran modified with sodium hydroxide with magnetic iron oxide of a colloidal size and in U.S. Pat. No. 4,452,773, magnetic iron oxide microspheres obtained by covering magnetic iron oxide particles having a particle diameter of 10 to 20 nm with dextran. Further, complexes of carboxyalkyl ethers of polysaccharides with magnetic iron oxide having a particle diameter of 2 to 100 nm are disclosed in Japanese Patent Application Kokai (Laid-Open) 134001/1991 (U.S. Pat. No. 5,204,457). These complexes are useful as an MRI contrast medium, particularly an MRI contrast medium for liver but have several points to be improved in order to use them as an MRI contrast medium for other organs or sites.
Accordingly, the present investors have prepared a complex of a polysaccharide derivative having both substituents of a carboxyalkyl group and an unsubstituted or substituted aminoalkyl group with a magnetic metal oxide and administered it into a blood flow in the form of an aqueous sol to find that it has a slow blood clearance (slow removal from blood) and a low toxicity and is very useful as a nuclear magnetic resonance imaging (hereinafter abbreviated as MRI) contrast medium, particularly a contrast medium for a blood vessel, and thus have come to complete the present invention.